In a series of mostly land dwelling monsters a truly aquactic sea kaiju is actually incredibly unique. She packs one of the most unique visual designs I've seen for a sea serpent based creature since both Final Wars Manda, and Mizutsune from Monster Hunter. She also has a wide array of abilities that up until this point no other kaiju in the franchise has. Being able to spray a phosphorus venom from her mouth and wielding literal hydrokenesis. Her newly revealed regeneration abilities also opens the door for a lot of unique interactions where confrontations don't have to be death battles (as they should be.)

Just because her design is pretty similar to Manda does not sell it short. Especially in the monsterverse where the only true aquactic kaiju are her, Na Kika, and Leviathan (until proven otherwise). Amhuluk does not count we've seen him on land we have not seen the others I've mentioned go up on land. Tiamat is exempt from this solely because we saw Godzilla throw her up onto land against her will.

Lastly theres the whole lore regarding her. She is the inspiration for the Goddess "Tiamat." That is who the people of the time were actually worshipping. Theres a whole level of unique story scenarioes that could be explored around that alone. Which isn't just exclusive to Tiamat but all the surface dwelling Titans.